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Chapter 2
Atoms, Molecules and Ions
Atoms,
Molecules,
and Ions
Atomic Theory of Matter
• The theory that atoms are the fundamental building blocks of matter
reemerged in the early 19th century, championed by John Dalton.
• Each element is composed of extremely small particles called atoms.
•
•
Protons (+), neutrons (0), electrons (-)
Protons and neutrons in nucleus, electrons occupy energy levels (orbitals) outside
nucleus
• All atoms of a given element are identical to one another in mass and
other properties, but the atoms of one element are different from the
atoms of all other elements.
• Atoms of an element are not changed into atoms of a different
element by chemical reactions; atoms are neither created nor
destroyed in chemical reactions.
Atoms,
Molecules,
and Ions
• Compounds are formed when atoms of more than one element
combine; a given compound always has the same relative
number and kind of atoms.
• Law of Constant composition:
• Also known as the law of definite proportions.
• The elemental composition of a pure substance never varies.
• Law of Conservation of mass:
• The total mass of substances present at the end of a chemical
process is the same as the mass of substances present
before the process took place.
Atoms,
Molecules,
and Ions
• Dalton had no idea of atomic structure
• By mid-1800s scientists were experimenting with cathode ray
tubes (fig 2.3, 2.4). J. J. Thomson’s discovery of the electron
in 1897 showed that atoms have parts.
• In 1909 Robert Millikan measured the charge on an electron by
his oil-drop experiments (Fig 2.5).
• Circa 1900: J. J. Thomson’s plum pudding model of the atom
– The atom was a positive
sphere of matter with electrons
embedded in it.
Atoms,
Molecules,
and Ions
•
Discovery of the atomic nucleus in 1911 by Ernest Rutherford.
-- The gold foil experiment:
Rutherford shot  particles at
a thin sheet of gold foil and
observed the pattern of
scatter of the particles.
Atoms,
Molecules,
and Ions
The Nuclear Atom
•
Since some particles
were deflected at large
angles, Thompson’s model
could not be correct.
• Rutherford postulated a very
small, dense nucleus with the
electrons around the outside
of the atom.
• Most of the volume of the
atom is “empty space”.
• Protons were discovered in
1919 bt Rutherford; neutrons
in 1932 by James Chadwick.
Atoms,
Molecules,
and Ions
Subatomic Particles
• Protons and electrons are the only particles that
have a charge.
• Protons and neutrons have essentially the same
mass.
• The mass of an electron is so small we ignore it.
Atoms,
Molecules,
and Ions
Symbols of Elements
Elements are symbolized by one or two
letters.
Atoms,
Molecules,
and Ions
Atomic Number
All atoms of the same element have the same
number of protons:
The atomic number (Z)
Atoms,
Molecules,
and Ions
Atomic Mass
The mass of an atom in atomic mass units
(amu) is the total number of protons and
neutrons in the atom.
Atoms,
Molecules,
and Ions
Isotopes:
• Atoms of the same element with different masses.
• Isotopes have different numbers of neutrons.
11
C
6
12
C
6
13
C
6
14
C
6
Average Atomic Mass:
•Because in the real world we use large amounts of atoms and
molecules, we use average masses in calculations.
•Average mass is calculated from the isotopes of an element
weighted by their relative abundances.
Atoms,
Molecules,
and Ions
Periodic Table:
• A systematic catalog of
elements.
• Elements are arranged in
order of atomic number.
• Rows are periods,
columns are groups.
• Elements in the same
groups have similar
properties.
Atoms,
Molecules,
and Ions
Periodicity
When one looks at the chemical properties of elements, one
notices a repeating pattern of reactivities.
Atoms,
Molecules,
and Ions
Groups
These five groups are known by their names.
Atoms,
Molecules,
and Ions
Arrangement of Periodic Table
(see slide 12)
• Nonmetals are on the right side of the
periodic table (with the exception of H).
• Metalloids border the stair-step line (with the
exception of Al and Po).
• Metals are on the left side of the chart.
Atoms,
Molecules,
and Ions
Chemical Formulas
The subscript to the right
of the symbol of an
element tells the number
of atoms of that element
in one molecule of the
compound.
Atoms,
Molecules,
and Ions
Molecular Compounds
Molecular compounds
are composed of
molecules and almost
always contain only
nonmetals.
Atoms,
Molecules,
and Ions
Diatomic Molecules
These seven elements occur naturally as
molecules containing two atoms.
Atoms,
Molecules,
and Ions
Types of Formulas
• Empirical formulas give the lowest
whole-number ratio of atoms of each
element in a compound.
• Molecular formulas give the exact
number of atoms of each element in a
compound.
Atoms,
Molecules,
and Ions
Types of Formulas
• Structural formulas show the
order in which atoms are
bonded.
• Perspective drawings also
show the three-dimensional
array of atoms in a
compound.
Atoms,
Molecules,
and Ions
Ions
• When atoms lose or gain electrons, they become ions.
 Cations are positive and are formed by elements on the left
side of the periodic chart.
 Anions are negative and are formed by elements on the right
side of the periodic chart.
Atoms,
Molecules,
and Ions
Ionic Bonds
Ionic compounds (such as NaCl) are generally
formed between metals and nonmetals.
Atoms,
Molecules,
and Ions
Writing Formulas
• Because compounds are electrically neutral, one can determine
the formula of a compound this way:
 The charge on the cation becomes the subscript on the
anion.
 The charge on the anion becomes the subscript on the
cation.
 If these subscripts are not in the lowest whole-number ratio,
divide them by the greatest common factor.
Atoms,
Molecules,
and Ions
Common Cations
Atoms,
Molecules,
and Ions
Common Anions
Atoms,
Molecules,
and Ions
Inorganic Nomenclature
• Write the name of the cation.
• If the anion is an element, change its ending to -ide; if the anion
is a polyatomic ion, simply write the name of the polyatomic ion.
• If the cation can have more than one possible charge, write the
charge as a Roman numeral in parentheses.
Atoms,
Molecules,
and Ions
Oxyanions
•
Oxyanions are polyatomic ions containing one or more oxygen atoms.
•
When there are two oxyanions involving the same element:
 The one with fewer oxygens ends in -ite
• NO2− : nitrite; SO32− : sulfite
 The one with more oxygens ends in -ate
• NO3− : nitrate; SO42− : sulfate
•
When there are four oxyanions involving the same element:
 The polyatomic ion with the fewest oxygens has the prefix hypo- and ends
in -ite
Ex: ClO− : hypochlorite
 The one with the most oxygens has the prefix per- and ends in –ate
Ex: ClO4− : perchlorate
 The one with the second fewest oxygens ends in -ite
Ex: ClO2− : chlorite
 The one with the second most oxygens ends in -ate
 ClO3− : chlorate
Atoms,
Molecules,
and Ions
Acid Nomenclature
• If the anion in the acid
ends in -ide, change
the ending to -ic acid
and add the prefix
hydro- :
 HCl: hydrochloric acid
 HBr: hydrobromic acid
 HI: hydroiodic acid
Atoms,
Molecules,
and Ions
Acid Nomenclature
• If the anion in the acid ends in
-ite, change the ending to -ous
acid:
 HClO: hypochlorous acid
 HClO2: chlorous acid
Atoms,
Molecules,
and Ions
Acid Nomenclature
• If the anion in the acid ends in
-ate, change the ending to -ic
acid:
 HClO3: chloric acid
 HClO4: perchloric acid
Atoms,
Molecules,
and Ions
Nomenclature of Binary Compounds
• Contain two nonmetals
• The less electronegative atom is
usually listed first.
• A prefix is used to denote the number
of atoms of each element in the
compound (mono- is not used on the
first element listed, however.)
• The ending on the more
electronegative element is changed to
-ide.
 Carbon dioxide, CO2
• If the prefix ends with a or o and the
name of the element begins with a
vowel, the two successive vowels are
often elided into one.
 Dinitrogen pentoxide, not pentaoxide
Atoms,
Molecules,
and Ions